Apparatus for neutralizing certain distortion of optical projections.



D. F. 'COMSTOCK & 0. E. CONKUN. APPARATUS roa nzummzms 051mm msroanuu or OPTICAL PROJECTIONS.

APPLlCATiON FILED JULY 2419l6- Patented Nov. 5, 1918.

UNITED sra'ri s rn rnwr omen.

DANIEL F. OOMSTOCK, OF BROOKLINE, AND OLIVER E.

CO'NKLIN, OF BOSTON, MASSA- CHUBETTB, ASSIGNORS TO KALMUS, COMSTOCK & WESCOTT, INC., 01' BOSTON, MASSA- CHUSETTS, A CORPORATION OF MASSACHUSETTS.

APPARATUS FOR NEUTBALIZING CERTAIN DIS'IORTION 0F OPTICAL PROJECTIONS.

Patented Nov. 5, 1918.

Application filed July 24, 1916. Scria1No.110,818.

Projections, of which thc loll umu' ls u specificatwn.

This invention relates to ticlu llttrlt lt ll apparatus employed for projecting images on a screen which is oblique to th optical axis or to the central or average ray from the proicct r, and more particularly to a toric lens, or its equivalent, for overcoming keystone distortion without introducing astigmatism.

In the art of optically projecting upon a screen a magnified image of the ihotograph or other transparency. it has ecomc the custom to )lace the optical lantern above the level of the point upon the screen which coincides with the middle of the picture, in order that the lantern may occupv less valu ahle space in the theater or hall than the mint immediately opposite the screen which is usually required for seating purposes.

It is also customary to place the screen in an upright position substantially perpendicular to the normal line of rision to the average siectator. The resulting obliquity of inci once of the projection beam produces a screen known as the keystone elfcct. so called because a reetangularfigure at the object or transparency is distorted in the projected image to a figure resembling an inverted kcyst0ne. This key'tone effect is due to the fact. that all parts of the screen where it intersects the rays are not etpiidistant from the projection lens, in otier words, that the central ray is not normal to the screen; and since the rays of the beam leaving the projector diverge from the projector, the image on those parts of the screen more remote from the projector will be magnified to a. greater extent than those parts nearer the projector. Horizontal arallel, lines in the object remain parallel 1n the'im ige, while vertical parallel lines in the olnect appear in the image on the screen not as being parallel but as convergglass for ing toward a distant point in the plane of the screen and above the screen.

Moreover, with ordinary apparatus, the sl'iar niess of definition also suffers from this obliquity owing to the fact that the true image produced in space is in a plane normal to the optical axis while the screen is in a plane oblique to the optical axis. 'llheoretieally only that portion of the image is in focus on tluscreen which is defined by the inter-section oi the plane of the screen and the image plane.

For correcting the keystone distortion :1 tippcd cylindrical lens may be employed to modity or reconstruct the lens system in such a way that the image defined by a screen perpendicular to the axis'is no longer geometrically similar to the object but is distorted in such a way that it contains within it what might. be called an inverse keystone effect, that is, the distortion of the image beam is such as to show'a keystone effect of an opposite kind to that which the unmodified lens system would produce on the oblique screen at the same point. Such a system is disclosed in the application of Daniel 1!. Comstoch. Serial No. 35,196, tiled June 21, 1915. While such cylindrical lens is effective to correct the keystone distortion, it has a tendency under certain severe practical conditions to cause the projected images to be astigmatic, that is, the horizontal and vertical lines of the image under certain conditions will not he in focus at the same plane for the reason that drical lens acts as a plane parallel plate of the pairs of rays in a vertical )lane, and as a lens for the airs of rays in a. horizontal plane. Thus tile cylindrical lens tends to form two virtual images of the The principal object of our invention is to eliminate eystonc distortion and at the same time to avoid astigmatism so as to obtain sharp definition over the entire screen. This we accomplish by employing, in the optical axis of the beam of light and preferably between the object or transparency and the objective ens, retracting means comprisinga single lens, or a system of lenses, havin different refracting powers in vertical $11K horizontal planes, respectively. Suitable retracting means for the purpose may comprise two or more crossed cylindrical lenses, cemented together or spaced apart, one or more cylindrical lenses and one or more spherical lenses, cemented together or spaced apart, one or more spherical lenses, tOflOlGIl-SES, etc, the lens or lenses being tipped relatively either to the object or transparency, the virtual image thereof, the optical-axis of the tem or the central ray of the pro ected beam of light, the axis of the objective, or the image screen, or tipped relatvely to two or more of these.

In the accompanying drawings, I

Figure is a side elevation in diagram, partly broken away, showing the relation of projector and oblique screen in common use, which results in the keystone distortion of the projected image above described;

Fig. 2 is a face view in diagram of the,

keystone form of projected image caused by such distortion;

Fig. 3is a side elevation similarto Fig 1, showing in diagram a method of correcting such distortion; I v

Fig. 4 is a face view similar to b g. 2, showing the form of the corrected pro ected image;

Fig. 5 is an enlarged view of a portion of Fig. 3 looking in the direction of the arrow, showing one method of producing a distorted virtual iinage for counteracting the distortion of the projected image by means of a negative toric lens, I

F g. 6 1s a face view of the ob ect or transparency showing its relation to the virtual im e;

Fig. 7 is a face view of a double cylindrical lens; t

Fi 8 is a vertical sectional view of the doub e cylindrical lens;

Fig. i) is a horizontal sectional view of the double cylindrical lens;

Fig. 10 is a face view of a negative'gwric lens;

Fig. 1.1 is a vertical sectional view of the negative toric lens; and

Fig. 12 is a. horizontal sectional View of the negative torie lens.

Referring to Figs. 1 and 2, (i represents in conventional form the objective of an optical lantern, 1) represents the transparency and rreprescnts the screen image. It will be that the eentralline of thelight m from the objective to the screen is not normal to the screen, consequently the projected image 0 u the shape of an inverted keystone as illus' trated in Fig. 2, the lower part of the projected image a being, more remote from the optical lantern than the upper part, and therefore magnified to a renter extent. With the transparency or ob ect 6 normal to the central light beam 0!, the plane of the real image wi 1 also be normal to the central beam d, as shown at c. There will be a consequent loss of definition of the image a on the screen owing to the fact that the screen ima e 0 is not in the same plane as the plane of t e real image 0.

In order to, produce a distortion of the virtual. image to c'ounteractthe distortion of the projected image on the screen, a, refraeting element, or elements, is introduced between the transparency and the screen, preferably near the transparency, ca able of magnifying or reducing the virtue image more at the bottom or at the top and more in one direction than in the other direction, namely, more in the direction of distortion of the screen image, which magnification or reduction, or both, are qualitatively and quantitatively suitable for neutralizing the distortion of the screen image.

One suitable type of retracting elementmay be made by coi'iibinin with the single cylindrical lensdescribed 1n the/com anion application above referred to, a ve econt cylinon the screen will be in.

drical lens, which we call the corrector elemeat, positioned with its axis at ri ht angles to the axis of the first lens, wliit i we'call the keystone element. The eorrector element may be cemented on the side of the keystone clement away from the ob'ect, and when of proper strength it causes t e primary image to coincide with the secondary image. For best definition in the middle of the image the ratio of the power of the corrector .element to the power of the keystone element is the square of the cosine of the angle at which the lens is tipped relative to the beam of light; but for best average definition over rector olenient curved about a horizontal axis, the Hat faces of the two elements being cemented together. i

The toi'ie lens is another and a preferred type of lens for producing the compensating inverted keystone effect. 0 contemplate using either a positive or a negative toric essors lens, the positive lens luiving at least one surface like the LXit'tlflfl#tll'ffltr of a tour or torus shell. and the ltt'fi'iltifl: ions having at least one sin-lave like the interior surface of such a .--l|eii. A negati e torie ens ()f/ fifill alirhform. having hoth surt'arr.-1 voile-ave. is sliov'u iu F gs. il to 1;. 'lorir 5Eii have (liti'erefi't rurmturos in plane at angles to t'IHlt other. :tIlel toriitenses lln different power in these planes. Thus. in l igs. i l to 1; th lttititt t" is represented as having a shorter radius of i-lu'vutnre. in a horizontal plane than the radius of fEHRlllII'L, hen nsed as: a. kqwtoue loos. the horizontal section of the lens has thrpower neces znrv to prothn-e the inrei t; keystone etl't-rt'. and the vertical sleetion has. the power neressary to bring the primary and secondary images together. thus the horizontal and vertical sections of the torie lens perform. respectively. the flint? tions of the logvstouo und t'ol'l't't'tot elements of the crossed cylindriral lens romhination.

Figs. 1', 5 and ti illustrate the effect. of a, negative toritlens when employed ink one of the man ways within the scope of our invent ion. When using a tuFgative torio lens the lens should he tipped forward relative to a normal to the optiral axi as shown.

but when using a positive torie lens the lensshould he tipped in the opposite tlll'flt'lilUlt. In either wise a distorted virtual image similar to hut opposite to that of the screen in'iage is prodm-ed. a indicated diagrinnmaticallv at 31. he ohjeot r: may or may not he obliquito'the optirul axis. but as shown in Fig. 3 it is inolined harkwardly at an angle of three (ie;}flte= to the normal to the axis. 'lhus. in the modifiraiion illnstinted. hoth tin ohjott and the roinpensatlng lens ar oblique to the optical axis. However. it. nithin the srope of our invention to tip riiller the one or the other without tioniug both.

The parts at the lUl'lt lens L nearest the object it produce a certain magnification of the virtual image. depending on the tiistnnt'e, of the lens from the ohjert and on th focal length of the lens. The parts of the lens L at a greater distanro from the ohjet-t J; resuit in a de 'reased magnili .iin and in a removal of parts ol the virtual image fan the! from the transparency. Ev suitably choosing the strength of the lens (foral length} and the tip or inrlinaliou of the lens to the transn t-envy. any reasonable magnitude Hill V be given to the progressive nmgnifit-atiim of the virtual image. In other words. the distorti n eflert V\'lll('l1 is desired may he fi tttllnttl to any do irrd degree.

The relation of the olriert. and virtual image are illustrated in Fig. 5. wherein the distant-e of the lens i; from the oliieot 3), also the inclination of the lens L, and also the distance between ohjert and virtual image. are exaggerated for the purpose of elearness. Assume a heinn of light passing through point lot the object I). of whit-h one ra passes through lens L and is refrarted alongthe line r. while another ra v isrefronted along the line f. The virtual image of the point 1. ivill therefore appear at. point I which is the intorsert'ion of the projec tion of the .two lines P and f. Qhuilarlv, lie-ants passing through a point of the object l1 iinnunliately below the point 1 (indirat d at :3 in Fig. 6) iii pass along lines in the same vertical planes with e and f. through the lower part of the lens L, and he refracted along lines (F andf. The virtual in'lago of the point 2. therefore. will he at the inter- ;sert'ion of projections of the lines and f shown at $3. The other corners Send 4 of the object I) in a similar manner produce virtual images 3 and 4'. The virtual image i is thus distorted and when projected upon the screen by a beam oblique. to the screen appears as an undistorted image (Fig. 4) heeause the distortion of the virtual image If is equal and opposite to the distortion of the screen image due to the obliquity of the beam and therefore neutralizes it.

A suitahle torie lens is one having a power of -10 diopters in the horizontal sections and -8 diopters in the vertieal sections with the lens tipped forward. as shown in Fig. 3. 18 degrees and the film gate tipped baole ward 3 degrees. VVhr-n using a negative lens an erect positive spherirallens Z may be employed to millet-t the rays emerging from the negative lens. but when using a. positive lens an erert negative lens may be used to disperse the rays emerging: from the positive lens.

it will he observed from the above disclosure of certain illustrative forms of our invention that one g neric feature of the in vention is a lens having different powers in planes at. right angles to each other. This feature is present in lenses of various forms. For example. the lens may have curvature of one degree in one direction on one side and curvature of a difi'erent degree in the other direetion on the other side, as the douhle crossed eylindrical lens; or. a different degree in the other direction on the some side. as a lens having, one toric surface and one that or spherical surface; or the lens may have one degree of t'nrvature in one direction on one side and a difierent degree of curvature in the other direction on the same side. ronstituting a torie surftice. and have a similar (although not necessarily the same) surface on the opposite side. as the negative tori lens shown in Figs-1. 10. 11 and 12. Furthermore. the lens may he convex on one or both sides.

The words horizontal and vertical as used throughout the specification and claims r1 gesvlwl apparatus comprising means r g nn obgutt, :i. screen tipped rela- W nobj? an objective lens between :a'm iiw xEL-UH, and a refracting 5 file iiii'ociimi a! right i; i v nmm uuingtmf: aim-qua] i-nrvat1u'es, one plane PL'YiNIliiiiilitiT in H20 myth. I w mm: :1 Ffilffli arms and the other about 3.

mama? axis, the uivmunt bein tipped i'iviy m the nhjafl', whereby ieystone 'iiun vw'rv-ctml Without, introducing liiiibifnsi'igmafiism Pr'njiviifii'i :gpgmrat for pzY'rducing on :wi: iiiiiiii is uiilii' iie in the object, a. .i, i:mtiuiy undistnrted image im and in such manner that an Gbliqne n Hf the ifimgu imam in the. image reiurm-siily magnified uniformly. "km apparatus for pmdueing 0n iRi1 1 is @hiiqmz t0 the abject, a .ililiisiiiy nmiismrted image of in;- mmms fer prfijfsctin sin: abject to the screen, nd? jig szu'fzice nirvana-e both and vm'fimi iialles and being 5', {v m the rabient fedistorting a E: manner as $0 produce in a J i rho, igwnm an image of varv- 241mm W lfrgiifiratirm 0f rim-h extent that ugiaifiiratmn on the oblique piziiiw if?" :iiuu appnrziiiis fur prmiucing on is ohiiqm; to the object, a 180 i a zisianiizaiig uni-Haunted image of risiiag IIIPHUS for projenting i m n ihn (iiJjPft tn the screen, an" :1 curvature both in and mirth-a5 pimwa and being flaw-i; ii; the npi *1 axis for dis- :iiw Mum in and; manner as to profile beam an image magniiimion of such ex- -i zimg'niiivation 0n the ES :mifmm.

! iipllfiiiiiliifi for producing on is; ubiiqam In the, object, a, -miiaii y nmiisim'iicd image of I win-y, kmpiisirig means for pro- 5mm Eight from The object to T5, :in means fin, refraeting the mfwmniniiiy witiimit introducing 1 am! 2:: such manner as is proa virtual image of the transparency distorted in a manner and to an extent propon tional and opposite to the normal distortion of the projected in'ia e (lilu to the obliquity of the screen. k

13. Projection apparatus for producing a magnified, substantially undistorted image of a trans iarenav on a screen which is oblique to the central my oi the beam of light strik ing the sin-ecu, comprising a torie lens tipped relalivo n i. {.ranspan-nry for refraeting the rays o-E' in, it jlllfiilll; an. the objeet to the S(.ll"i. nlwrib ihrri lwi pro-tinned in a plane nornnii boa ii an image of varying lateral niag'ii'xheafimi sueh that he absolute magnification on rho O illlfle srreen is uniform.

14.. Projection aj'iparalus comprising an objective lens and reflecting means having one operative surface curved. in one direction and having a second o 'ierative surface curved in a direction at right angles to the first surface. the degrees of curvature of the two surfaces being different, the refraoting means being oblique to the virtual image produced thereby, whereby an image sub stantially free from keystone distortion may be produced in plane oblique to the axis of the objective lens.

15. Projection apparatus having an optical axis including a fihn gate, an objective lens, a screen. and rrl rlu-l ng means having one surfaee eurveii about a horizontal axis and another surfaee eniw ed a greater amount about a vertical axis. the screen and the refraoting means being tipped relatively to the film gate about. a horizontal 16. Projection apparatus adapted in projeoton a screen an inia free from keystone distortion i-on1prising an objective lens and a toric lens. the l'orii: lens being in the optical axis of the objective lens and having one axis at. an oblique angle to the optical axis of the objeetive lens, the i'orio ions having greater power in one (.lireotion than in a direction pi-rpeniliuular to the first ilirers tion.

17. Projection apparatus adapted to pro joo on a screen an image free from keystone distortion comprising an objective lens and a loric lens in the optical axis of the objective lens. the torlo lens being so disposed at an oblique angliwith the optical axis as to eliminate the keystone distortion.

I lfrojurlion apparatus adapted to prnii-vi on a screen an Lama's". free from keystone ll i)l'lill!|.(tillljli'lhillgilnUbjQCtlYl! lens and a t ri lens in and obi'iquo in the nptiijal axis of the o iji-ri ir lens, anll means For suppolling llu ,iliji-ui i1; is: jwteri in a plane obliipnto [luoptiial axle. llio obliquity of llw ohji-ei Hui loriw l lli eing so Man-ii w; in i'limiumn 'ovsiinn} lisi'iriirnn T l'im, :iiiaptwl lo pr ji r on a. serum an 11.1; we from keystone lion .TiEHHiiT'iE distortion, comprising negative lens means for ('nlnpt'nsating for keystone distortion, and a positive spherieal lens for collecting light emerging from said lens means.

20. Projection apparatus having an optical axis andan object, a negative toric lens, a positive spherical lens, on objective lens, run] a srroun, the toric lens and the screen be ing oblique to the optical axis.

531. irojeciion apparatus wherein the screen aml the object are oblique to the optimal axis, having an objective lens, and a. litranting eieinent oblique to the optical axis for preventing keystone distortion and as tigniatisin.

Projeetion apparatus, wherein the screen oblique to the optical axis, having an objective lens, and a refraoting element oblique to the. optical axis for preventing keystone rlisi'orl'ion and. astigmatism.

23. Projection ap liaratus adapted to 1'0- joet on a screen an image free from eystone distortion having an optical axis including a negatinluric lens inclined at an angle to the optical axis, and a positive spherical lens.

Projection apparatus adapted to project on a screen an image free from keystone distortion comprising an objeetive lens and a torie lens in and oblique to the optical axis of the objective lens.

Projection apparatus adapted to ro jeei. on a. soreen an image free from eystone distortion comprising a toric lens and an object, both within and oblique to the 0pdistortion comprising a lens system, toriq in character Within the optical axis and tipped relatively to the transparcl'iry.

28. Projection apparatus having means for holding the object to be projected, a screen in a plane oblique to the plane of the ohjeet, and a retracting element having different ourvalure in horizontal and vertical directions respectively, the refracting element being between the object and the screen.

29. Projer'rion apparatus having means for holding the. object to be projected, a semen in. a plane oblique to the plane of the object, and a lens between the object and the screen, the lens having a surface curved a greater amount in one ilirection than in the other direction.

lrojorl ion apparatus adapted to project on :1 error-n oblique. to the optical axis an image free from keys-mun distortion comprisi 'an obj-wt obiilpulo the optical axis, and i'eflaviiug means in the path of the projected image having different refructing powers in horizontafl and \m'tical directions respev tively.

31. Projection :lppnrntus adapted in PTO- B je ct on a servvu an image free from key stone distortion mmpl-ising moans fur huid in the 0bje rt to he mljvcted in a plane 0b ique t0 the optical axis. and a lens system in the path of the proj'ected image having diffel'vnt. powers in horizontal and vertical 10 planes rvspectively. I

Signed by us at Boston. Massachusetts, ihis 15th day of July, 1916.

DANIEL F. (.OMSTOCK. O. CONKLIN. 

